Close to 5 inches

The screen itself is 5 3/8 inches in diameter.

Hi Mark the rating on this funnel screen is 400 micron.

The screen is really fine. I cannot give you a measurement but I use this funnel and screen to filter the wort when I transfer the wort from my mash tun to my boil kettle. It produces pre-boil wort with no grain particles in it. I Vorlauf first to set the grain bed and then use this filter when transferring the wort. Works really well.